1. Field of the Invention
This invention relates to a high-polymer gel and a blood vessel obstructing material comprising the same. More particularly, this invention relates to a vascular lesion embolizing material that can advantageously be used for the treatment of vascular lesions such as aneurysms and arteriovenous malformations (AVM).
2. Prior Art
In order to increase the number of successful cases in the treatment of cerebrovsacular lesions such as a large arteriovenous malformation which is difficult to access anatomically and a large aneurysm which defies surgical operations or which is amenable to surgical operations only with much difficulty, and for the purpose of reducing the physical, mental and economic burdens on patients, there has recently been a great enthusiasm for intravascular surgical treatments which are conducted using intravascular catheters without casing stresses due to surgical operations (Katsuya Goto, Neurosurgeons 9, 229-239, 1990).
In the case of treatment of a cerebrovascular disorder, a very small catheter is inserted superselectively into the affected site of a cerebral artery and an embolizing substance is supplied through the catheter so as to obstruct a cerebral aneurysm, an arteriovenous malformation, an arteriovenous fistula, and the like.
The blood vessels of the brain are different from the blood vessels in other parts of the human body in that they lack an outer elastic membrane and that they have thin walls; therefore, they are less resistant to the lateral pressure of blood streams. Furthermore, blood vessels run in a complex way within the skull and, in particular, patients suffering from hypertension tend to experience disorders under stresses acting on branches. Autopsy statistics teaches that in one out of a hundred adult cases, cerebral aneurysms occur in the arteriovenous system; they have a wide morphological distribution ranging in diameter from about 1 mm to about 20 mm and more, with the site of their occurrence centering on the circle of Willis but distributed widely among cerebral arteries. If cerebral aneurysms rupture, they will cause serious cerebrovascular disorders such as subarachnoid hemorrhage and intracerebral hemorrhage; if they grow in size excessively, they will cause a symptom of compressed cranial nerves. An arteriovenous malformation is the most common and best known of cerebravascular malformations and it consists of an aggregate of meandering or dilated blood vessels to and from the brain and intervening blood vessels that have arteriovenous anastomosis. Clinically, areteriovenous malformations are important as a cause of intracerebral hemorrhage, subarachnoid hemorrhage, epilepsy and progressive neurofunction loss that are manifested in fairly young patients.
Embolization is a technique that is intended to treat the cerebral arterial lesions described in the preceding paragraphs by obstruction with embolizing substances so that the blood stream in the lesion is suspended to coagulate the affected part. If necessary, the coagulated part may be removed.
The embolization technique has come to play a central part in the treatment of cerebral arteriovenous malformations (Goto, K. et al., Neuroradiology 33 (Supple) 193-194, 1991) and so will it be in the treatment of cerebral aneurysms in the near future (Katsuya Goto, Igaku no Ayumi (Advances in Medicine) 153: 653, 1990).
Cyanoacrylate base materials have heretofore been used as common liquids for obstructing sites of vascular lesions (J. Biomed. Mater. Res., 17, 167-177 (1983) by M. C. Harpers et al.).
It has recently been proposed that a solution of an ethylene=vinyl alcohol copolymer (EVAL.RTM.) in dimethyl sulfoxide (DMSO) be used as an embolizing material so that DMSO is diffused in blood to obstract a blood vessel by precipitating EVAL.RTM. (Medical Tribune, Oct. 26, 1989, pp. 46-47).
Another method that is practiced today is to used a balloon (detachable balloon) that can be cut off within an aneurysm; the balloon is inserted into the aneurysm, blown to obstruct it using a catheter and thereafter detached to be retained in it (Journal of Neurosurgery, 41, 125-145 (1974) by F. A. Serbinenko).
Metal coils (minicoils), polyvinyl alcohol (PVA) sponges, alcohols, sutures, etc. have also been used in accordance with the specific objective of treatment.
The conventional embolizing materials and methods, however, have various problems. First, the cyanoacrylate base embolizing substances are difficult to inject into blood vessels since they will rapidly solidify to polymerize. In order to insure that a catheter will not be pasted to the inner surface of a blood vessel in the brain on account of the strong bonding action of the embolizer, the catheter must be pulled out of the sheath as quickly as possible after injection of the embolizer ends. This adds to the difficulty encountered with handling of the conventional cyanoacrylate base embolizing substances, and even if occlusion of the blood vessel by first injection is found to be inadequate, second injection is impossible. A further problem with these embolizing substances is that their irritating action on the wall of a blood vessel is strong enough to potentially cause an intense inflammatory reaction.
The system having EVAL.RTM. dissolved in solvent DMSO has the problem that the solvent will linger in the central part of EVAL.RTM. that has precipitated upon contact with running blood. In the case where the system is used for embolization of an arteriovenous malformation, the substance that has precipitated within the blood vessel tends to be disrupted into small pieces by turbulent flows of the blood, which will be carried away in veins coming out of the brain. If the system is used for embolization of an aneurysm, the part of the precipitate that projects beyond the exit of the aneurysm stream in the Y-shaped branch of a blood vessel as shown schematically in FIG. 2.
As a further problem, DMSO is not an ideal solvent since its safety has not yet been established (Hiroo Iwata et al., Preprint for the 11th Meeting of the Society of Biomaterials of Japan, 68, II--22, 1989) and, in addition, it can do harm to apparatuses that are made of plastics.
A detachable balloon is frequently used in embolization of aneurysms. However, because of the weak reaction between the balloon and the inner surface of the aneurysm, the cavity in the aneurysm might not be completely filled with the balloon and no matter how small the unfilled lumen may be, there is a high risk of the recurrence of the aneurysm. If it is attempted to fill the lumen as much as possible by maximizing the inflation of the balloon, the pressure buildup in the latter will alter the shape of the aneurysm, increasing the change of its rupture. As a further problem, a tractive force must be applied to detach the balloon from the catheter but this again increases the chance of the rupture of the aneurysm.
The state-of-the-art technology of embolization is such that it is difficult to block the neck of the aneurysm completely even if a detachable balloon or a minicoil is used and proximal obstruction (part of the parent artery is obstructed) is more often practiced (Akira Takahashi, Igaku no Ayumi (Advances in Medicine), 154, (7), 432, 1990). However, this method unavoidably sacrifices the parent artery, exposing the brain to the danger of a blood stream disorder. It has been proposed that a detachable balloon be used in combination with minicoils for embolization of a large aneurysm (Katsuya Goto, Abstracts of the IVR Study Group of Angiography, Japan, 1991); however, this method requires many expensive minicoils, takes much time and yet it is incapable of achieving the intended evacuation.
Other conventional embolizing materials such as PVA granules, alcohols and satures are not only difficult to handle, they also have many problems in association with treatment such as insufficient therapeutic effects.